There is known a valve device for an exhaust gas flow path to open and close a communicating flow path that communicates an upstream chamber and a downstream chamber with each other in an exhaust gas flow path for an in-vehicle internal combustion engine. In Patent Document 1 for example, there is disclosed a valve device disposed in a muffler for an internal combustion engine. An inside of the muffler is partitioned into an upstream chamber and a downstream chamber by a separator, and the valve device is provided to the separator to open and close an opening communicating the upstream chamber and the downstream chamber with each other. In the valve device, a valve body capable of closing the opening is supported by a support body so as to be rotatable about a rotation axis, and the valve body is biased in a valve closed direction by a coil spring.
When the number of revolutions of the internal combustion engine is low, an acting force exerted on the valve body by a pressure in the upstream chamber is smaller than an acting force exerted on the valve body by the sum of the biasing force of the coil spring and the pressure in the downstream chamber. Thus, the valve device is in a valve closed state in which the opening is closed. In contrast, when the number of revolutions of the internal combustion engine is increased, the acting force exerted on the valve body by the pressure in the upstream chamber becomes larger than the acting force exerted on the valve body by the sum of the biasing force of the coil spring and the pressure in the downstream chamber. Thus, the valve device is brought into a valve open state in which the valve body is spaced apart from the opening to thereby open the opening.